1. Field of the Invention
The present invention relates to exploration for sources of hydrocarbon fuel and particularly to enhancing the safety of oil and gas well drilling procedures. More specifically, this invention is directed to apparatus and methods for detection of the infusion of gas into a borehole and especially to apparatus and methods for a gas infusion detection system which is continuously operable during drilling for blowout protection.
2. Description of the Prior Art
In the drilling of oil and gas wells, drilling safety and efficiency are paramount considerations. Efficient operation of the drilling apparatus, particularly as wells are drilled deeper and offshore activity increases, demands that data of interest to the driller be collected downhole and be sensed and transferred to the surface "continuously", i.e., without the lengthly delays which would be incident to stopping drilling and lowering test instruments down the borehole. In recent years, significant advances have been made in measurement-while-drilling (MWD) technology. For examples of MWD systems for use in the measurement of borehole directional parameters reference may be had to U.S. Pat. Nos. 3,982,431, 4,013,945 and 4,021,774 all of which are assigned to the assignee of the present invention.
The measurement systems of the above-referenced patents utilize mud pulse telemetry to transmit information from the vicinity of the drill bit to the surface drilling platform. Mud pulse telemetry consists of the transmission of information via a flowing column of drilling fluid, i.e., mud, the information commensurate with the sensed downhole parameters being converted into a binary code of pressure pulses in the drilling fluid within the drill pipe which are sensed at the surface. These pressure pulses are produced by periodically modulating the flowing mud column at a point downhole by mechanical means, and the resulting periodic pressure pulses appearing at the surface end of the mud column are detected by a pressure transducer conveniently located in the standpipe. The drilling mud is pumped downwardly through the drill pipe (string) and thence back to the surface through the annulus between the drill string and wall of the well for the purpose of cooling the bit, removing cuttings produced by the operation of the drill bit from the vicinity of the bit and containing the geopressure.
As noted above, drilling safety is of paramount importance; and one safety problem relates to what is known as a "blowout". A zone of high geopressure, contained by cap rock, will occasionally be unknowingly encountered during drilling. If this pressure exceeds the hydrostatic pressure exerted by the drilling mud, and the formation has sufficient permeability to allow fluid flow, then the formation fluid will displace the drilling mud. This is referred to as a "kick"; and if unchecked will cause what is known as a "blowout" condition. One borehole condition which the driller desires to monitor, in order to ensure against "blowout", is gas influx.
While various techniques have previously been proposed, and in some cases implemented, for measuring gas infusion into a borehole, the previously proposed techniques have not been suited for MWD and have often been either complex, difficult to implement or have been comparatively slow. The prior gas influx measuring techniques have also often been incapable of providing unambiguous information thus requiring repeated tests and/or the use of plural measuring techniques. The methods of measuring gas influx into a borehole proposed in the prior art have included sensing the borehole annulus pressure, sensing the pressure differential between the interior of the drill string and the annulus, measuring the velocity of sound in the drilling mud, measuring the resistivity of the drilling mud and various other tests based upon attempts to measure the pressure of the formation through which the drill string is penetrating or has pentrated. As noted above, these previously proposed gas detection techniques and particularly those based upon pressure measurements, all have deficiencies which precluded their use in MWD and otherwise severly limited their usefulness.